The present invention is directed to the delivery of data via a wireless connection and, more particularly, to the accurate detection of data received via a wireless connection.
In a wireless communications system, data is delivered from a transmitter to a receiver using radio waves or other electromagnetic waves. The waves, however, may be reflected off objects in the environment and scattered randomly while propagating from the transmitter to the receiver so that multiple images of the transmitted signal may arrive at the receiver via different scattered paths. The scattering, known as multipath scattering, can cause the images to arrive at the receiver at slightly different times and interfere destructively, thereby cancelling each other. Thus, multipath scattering may significantly impede the accurate transmission of data over the wireless connection.
A known approach for achieving high spectral efficiencies is by transmitting the data using a base transceiver station (BTS) having multiple antennas and then receiving the transmitted data using a remote station having multiple receiving antennas. This approach is known as Multiple Transmit-Multiple Receive (MTMR). As an example, a single data stream is divided into multiple substreams, and an array of transmitter antennas is used to concurrently transmit the parallel substreams using the same frequency band. The transmitted substreams are then picked up by an array of receiver antennas. Each of the receiver antennas receives all of the transmitted substreams as a single, superimposed signal, rather than as a plurality of individual signals. A receiver located at the remote stations then demodulates the superimposed substreams to recover the original, transmitted substreams.
The scattered images of the transmitted substreams, however, are also picked up by the array of receiver antennas. Because each of the substreams originates from a different transmitter antenna located at a respectively different point in space, each substream is scattered differently, i.e. independently, and is thus received independently at each receiver antenna. When there is sufficient multipath scattering, the original substreams are not readily recovered.
The scattered images of the transmitted substreams may arrive at the receiver antenna at the same time, i.e. with no delay spread, known as flat fading or may arrive with at different times, i.e. with a delay spread, known as frequency selective fading. In the delay spread or frequency selective fading environment, currently transmitted symbols interfere with previously transmitted symbols, which is called inter-symbol intereference (ISI).
A known signal processing algorithm, referred to as “Bell Laboratories Layered Space-Time” (BLAST), uses differences in the scattering to identify and recover the transmitted substreams. The substreams are first sorted according to the strength of each channel, and the strongest substream is detected and extracted from the total received signal. Then, the substreams are again sorted based on their signal strength, and the next strongest substream is detected and extracted from the total received signal. The sorting and extracting process is repeated until all of the substreams are determined. The BLAST algorithm assumes that only flat fading is present and is therefore not suitable for delay spread environments, such as in a mobile wireless environment.
Thus, in the presence of ISI, the BLAST algorithm is not applicable. Moreover, while detecting the current strongest substream, the BLAST algorithm nulls the remaining substreams. This nulling is often imperfect because of channel estimation errors and, as a result, the performance of the receiver is degraded.
It is therefore desirable to recover the received data streams in the presence of ISI and without degrading receiver performance.